The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for compensating for patient motion during an MRI scan using ultrasound.
Respiratory motion is a major cause of image degradation in abdominal MRI. The artifacts caused by respiratory motion are often alleviated by the use of MRI navigators. In typical MRI navigator techniques, short imaging blocks are used to assess the current respiratory motion phase. These imaging blocks, referred to as navigators, are interleaved between imaging blocks that obtain the images of the subject. Examples of navigators include a pencil beam excitation that crosses the diaphragm, or an image of the liver region obtained using a bright-blood single-shot echo-planar imaging (“EPI”) pulse sequence.
Position information is derived from the acquired navigators. This position information may be used to retrospectively compensate for the effects of motion during the image reconstruction process, or to prospectively compensate for the effects of the motion by changing the excitation and readout parameters used during the on-going MRI scan. Navigator techniques can be classified as “gating” techniques if they reject unusable data, or as “motion correcting” techniques if they modify each data acquisition in proportion to the measured position offset. The position information derived from an MRI navigator can also be used to control another real-time system, such as a steerable focused ultrasound ablator.
Navigators often achieve good artifact reduction, but they also have significant costs. Navigators typically slow down the imaging process, can interfere with the maintenance of steady-state magnetization, and require difficult pulse-sequence engineering that may need to be repeated for each combination of navigator type and imaging sequence. It would therefore be desirable to provide a system and method for compensating for subject motion during an MRI scan that overcomes these limitations of relying on MRI navigators.
A number of papers have demonstrated that pulse-receive ultrasound devices can be operated in the MRI environment, and that ultrasound may be used for subject motion compensation. These studies that used ultrasound to compensate for subject motion in MRI disclose directly measuring positions within the subject using ultrasound. In one approach described by P. L. de Oliveira, et al., in “Rapid Motion Correction in MR-Guided High-Intensity Focused Ultrasound Heating Using Real-Time Ultrasound Echo Information,” NMR in Biomedicine, 2010; (23):1103-1108, a pencil-beam ultrasound transducer was oriented so that the direction of the subject motion was along the axis of the ultrasound beam. In this method, position information was directly computed from shifts observed in the echo delay. The authors of this technique noted that it was not expected to work in vivo because organs, such as the liver, predominantly move in the craniocaudal direction, a direction with which an externally placed ultrasound transducers cannot be aligned, as required by this method.
An attempt to solve the problem of tracking motion along an inaccessible axis was described by M. Pernot, et al., in “3-D Real-Time Motion Correction in High-Intensity Focused Ultrasound Therapy,” Ultrasound in Medicine & Biology, 2004; 30:1239-1249, in which the direct shift-tracking technique was extended to use three or more ultrasound transducers. In this technique, the ultrasound transducers were widely spaced so that their beams were oriented toward a focal point at large relative angles. The direction of motion was then determined from the shift observed in each transducer. This technique has the advantage of direct displacement measurement, but requires expensive multiple-transducer transmit-receive capabilities, is intrinsically limited to measure simple translations, and will accumulate errors due to velocity integrations in the position estimate, resulting in a drift in the position measurement over time. This technique has not been demonstrated in conjunction with MRI.
A technique that attempted to permit measurement of non-translational motion and to avoid problems associated with cumulative estimators was described by M. Gunther and D. A. Feinberg in “Ultrasound-Guided MRI: Preliminary Results Using a Motion Phantom,” Magnetic Resonance in Medicine, 2004; 52(1):27-32. In this technique, a linear ultrasound transducer array was used to produce two-dimensional ultrasound images. The position is indicated directly by shifts and rotations observed in these images. This approach was recently demonstrated for in vivo motion compensation in cardiac imaging, as described by D. A. Feinberg, et al., in “Hybrid Ultrasound MRI for Improved Cardiac Imaging and Realtime Respiration Control,” Magnetic Resonance in Medicine, 2010; 63:290-296. Although this technique is capable of tracking motion along the craniocaudal axis, it requires a substantial investment in ultrasound equipment and electronics. The technique is also limited to measuring motion that occurs within the ultrasound imaging plane, and will not detect small displacements in the through-plane direction. These small displacements that occur in the through-plane direction are important for interventional applications.
Thus, it is desirable to provide a system and method for motion tracking and compensation in MRI and other clinical applications using ultrasound such that motion along multiple different motion axes can be tracked.